1. Field of the Invention
The present invention relates to a rotating machine, which is capable of controlling a rotational speed or a rotational angle of a rotatable shaft or of a rotated body connected to the rotatable shaft.
2. Description of Related Art
For example, an image processor (image forming apparatus), such as a color copying machine, a color printer, has respective photoconductor drums of four colors (black, yellow, blue and red). Each of these photoconductor drums is rotated at a low speed (40 rpm to 100 rpm) by a corresponding electric motor, which serves as a rotating machine. In the motor for rotating and operating the photoconductor drum, it is desirable to eliminate rotational irregularities (rotational speed fluctuations), which cause a deterioration of an image quality, at the above low rotational speed.
Therefore, an encoder, which senses a rotational speed of a rotatable shaft of the motor or of a connecting shaft of the photoconductor drum connected to this rotatable shaft, is provided. The rotational speed of the photoconductor drum (the motor's rotatable shaft or the connecting shaft) is controlled based on an output signal of the encoder. An optical type encoder has been adopted as the above encoder. In this encoder, for example, an encoder plate, which forms an optical pattern constructed by multiple slits arranged at equal intervals in the circumferential direction, is coaxially attached to the above rotatable shaft. A light emitting element and a light receiving element (hereinafter collectively called a rotation sensing device) are arranged such that the optical pattern is interposed between the light emitting element and the light receiving element. The rotation sensing device outputs a pulse signal (ON/OFF signal) according to the presence of light reception caused by rotating the encoder plate.
A construction, which includes two rotation sensing devices provided to a single encoder plate, is known to improve rotational detection accuracy using the encoder (e.g., see JP-A-7-140844). Specifically, as shown in FIG. 11A and FIG. 11B, the encoder 200 includes an encoder plate 204, multiple slits (an optical pattern) 204A and two rotation sensing devices 206, 208. The encoder plate 204 is coaxially secured to a rotatable shaft 202 and is rotated integrally with the rotatable shaft 202. The slits 204A are arranged at equal intervals in the circumferential direction in the vicinity of the outer circumference edge of the encoder plate 204. The rotation sensing devices 206, 208 are arranged at two points, which are symmetrical with respect to the axis of the rotatable shaft 202.
A control device, which is electrically connected to the encoder 200, is constructed such that output signals of the rotation sensing devices 206, 208 are respectively inputted to the control device. An influence, such as an attachment error (eccentricity) of the encoder plate 204 relative to the rotatable shaft 202, is removed by averaging these output signals. Namely, since a rotational detection error, which is caused by the above attachment error and has a sine wave form, occurs once per rotation of the rotatable shaft 202, the influence is removed by averaging the output signals of the two rotation sensing devices 206, 208, which are arranged in the corresponding positions opposed to each other by 180 degrees. Thus, in the above construction (rotation detecting method) having the encoder 200 and the control device, the true rotational speed (angular velocity), from which an error component caused by the above attachment error is removed, is obtained.
For example, when the encoder plate 204 is made from inexpensive polyethylene terephthalate (PET), a distortion could occur due to a difference between longitudinal and transversal expansion coefficients in this encoder plate 204, so that the encoder plate 204 is deformed approximately in an elliptical shape. This deformation of the encoder plate 204 made of the PET becomes notable under a high temperature environment of about 70 degrees Celsius. Since the rotational detection error, which is caused by such deformation of the encoder plate 204 and has the sine wave form, occurs twice per rotation of the rotatable shaft 202, this rotational detection error cannot be removed by the above conventional technique. Therefore, in the encoder 200 used under the high temperature environment, it was conventionally necessary to take a countermeasure using, for example, the expensive encoder plate 204 made of glass.